PROJECT SUMMARY Many infants in the United States and around the world grow up in bilingual environments, and their experience is defined by caregivers' dynamic switching between two languages. Contrary to popular belief, only a tiny minority of bilingual children grow up in a strict ?one-person-one-language? environment. Instead, most bilingual children regularly hear two languages from the same person, within the same conversation, and often within the same sentence (e.g., ?Look at the perro!?). Little is known about how young bilinguals learn through the inherent alternations between languages. Our primary goal is to understand how bilingual infants and toddlers learn two languages in the context of everyday switching across sentences, conversations, and people. Based on findings from our previous NICHD-funded R03, we will test the overarching hypothesis that language switching in bilingual environments is a key contributor to bilingual infants' language learning and language outcomes. The proposed international, cross-lab project will test the same Spanish-English and French-English bilingual children at 12, 24, and 36 months of age, using complementary behavioral, household, and longitudinal measures of young bilinguals' learning from language switches. Aim 1 will use eye-tracking and pupillometry experiments to investigate how bilingual infants and toddlers process and learn from language switching across sentences (Exps. 1-2), conversations (Exps. 3-4), and people (Exps. 5-6). Aim 2 (Exp. 7) will use multi-day recordings of household language to investigate whether and how language switching at home shapes early language processing and contributes uniquely to language and cognitive outcomes. The proposed experiments will be conducted simultaneously with two distinct bilingual populations: Spanish-English bilinguals in New Jersey, and French-English bilinguals in Montral. Based on intersecting sociolinguistic, demographic, and experiential differences between these communities, this approach will address the crucial puzzle of generalizability in bilingualism research, i.e., whether findings are specific to one population or whether they warrant generalized conclusions. In summary, the proposed studies will illuminate an important, real-life challenge for language development: how bilingual infants and toddlers process familiar words and learn new words from the complexities of dual-language input, both from moment to moment and in aggregate. Findings will enable us to (a) generate a novel and comprehensive theoretical model of the emergence of bilingualism in infants and toddlers, and (b) create and disseminate evidence-based guidelines for fostering early bilingualism. Bilingual parents almost invariably assume that some bilingual environments are better than others, and ? like educators, pediatricians, and speech-language pathologists ? they strive to optimize children's dual-language learning and minimize the risk of language delays. Our complementary measures of the dynamics of bilingual input and learning may lead to better ways of supporting infants' and toddlers' pathways to bilingual proficiency.